The present invention relates mainly to an evaluation method and an evaluation apparatus for batteries, and particularly to a method for evaluating safety in the event of an internal short circuit caused, for example, by entrance of foreign matter or breakage of the separator, and an apparatus used for the aforementioned evaluation.
The battery evaluation tests for evaluating the thermal behavior in the event of an internal short circuit as a safety item of batteries such as lithium ion secondary batteries include, for example, the UL standard (UL 1642) for lithium batteries and a guideline (SBA G1101-1997 Guideline for Safety Evaluation on Secondary Lithium Cells) from The Battery Association of Japan. Such evaluation tests are utilized and the results of the tests are reflected in the development of various batteries. For example, in Laid-Open Patent Publication No. Hei 11-102729 (Document 1), the battery manufacturing process has been improved in order to obtain a high-capacity battery exhibiting excellent high-output characteristics and cycle characteristics and having a high level of safety. In Laid-Open Patent Publication No. 2006-313739 (Document 2), it is proposed to set a predetermined range for the ratio between the thickness of the heat-resistant porous layer disposed between the positive electrode and the negative electrode and the thickness of the side wall of the battery can, in order to achieve both a high level of safety and high battery characteristics in a lithium secondary battery.
The above-described evaluation tests include, for example, the nail penetration test, which is a test for examining an internal short circuit caused by a nail penetrated through or pierced into a battery from the side. By piercing the nail, a short circuit portion is caused by the nail between the positive electrode and the negative electrode inside the battery. Consequently, a short circuit current flows through a short circuit portion, thus generating Joule heat. A change in the battery temperature or the battery voltage that is caused by these phenomena is measured in the internal short circuit test.
The crush test is a test for examining an internal short circuit caused by physically deforming a battery using a round bar, a square bar, a flat plate or the like. More specifically, in the crush test, an internal short circuit is caused between the positive electrode and the negative electrode by physical deformation, and a change in the battery temperature or the battery voltage is measured.
However, with these conventional battery evaluation methods, it is not possible to accurately evaluate safety against an internal short circuit that may be caused in the inner part of the battery on the market, especially by entrance of conductive fine foreign matter.
More specifically, the battery safety in the event of an internal short circuit is greatly influenced by the amount of Joule heat generated at the short circuit portion. When V (V) represents the battery voltage, R1 (Ω) represents the resistance at the short circuit portion and R2 (Ω) represents the internal resistance of the battery, the amount of heat W (W) generated at the short circuit portion is expressed by the following equation:W=V2×R1/(R1+R2)2 
The above-described equation indicates that the amount of heat generation changes according to the resistance R1 of the short circuit portion. That is, it can be understood that the battery safety also changes according to the resistance at the short circuit portion.
For example, in the case where the electrode plate is formed by a current collector and a material mixture carried on the current collector, it is known that a portion on which the material mixture is applied, a portion on which metal foil is exposed, and so on are present on the electrode plate, and the resistance value varies with the locations of the short circuit. In other words, it can be understood that the battery safety in the event of an internal short circuit varies with the portions at which an internal short circuit has occurred.
In the conventional nail penetration test, a short circuit starts to occur from the outermost part of the battery. For this reason, in the case where a low resistant area, specifically, a current collector-exposed portion or the like in which there is no active material is provided at the outermost part of the battery, the results of the evaluation is greatly influenced by the short circuit resistance at the outermost part, so that the battery is evaluated to be safe.
However, in case conductive foreign matter has entered into the battery, there is a possibility that an internal short circuit may occur at any given location inside the battery, depending, for example, on the size, shape or hardness of the foreign matter, thus causing an internal short circuit at an electrical resistance different from that of the nail penetration test.
That is, the conventional nail penetration test can evaluate only a part of the safety against an internal short circuit that may occur on the market.
With regard to the conventional crush test, it has been found by analysis of the short circuit behavior during the crush test that plural points are simultaneously short-circuited at once in the battery. That is, the crush test also cannot accurately evaluate the safety against an internal short circuit that may be caused on the market, especially by entrance of foreign matter and the like.
Further, in considering the purpose of use of batteries, it is necessary to know the level of safety performance of each battery, such as “There is no heat generation” or “There is some heat generation”, in the event of an internal short circuit. However, with the conventional evaluation methods, the safety against an internal short circuit cannot be evaluated accurately, and therefore, the level of safety of batteries also cannot be determined.
For the above-described reasons, there is a strong demand for an evaluation method and an evaluation apparatus for evaluating the overall safety of batteries against an internal short circuit by performing a short circuit test at a given location in a battery, and for determining the safety level of batteries.